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The paper presents a short summary of the state of the art with respect to fatigue strength of reinforced and prestressed concrete structures. The simplification made in ordinary design evaluations are shortly discussed. Offshore concrete structures are exposed to an environment which is different from that compared with ordinary land structures. The special feasures of this difference will be discussed. Specially, the nature of the environmental loads causes a random load application in the structure. The paper will discuss how this may be tackled in design. Different methods used in evaluation the fatigue strength of offshore concrete structures are discussed and a proposal is made, on how to formulate a design criteria for offshore concrete structures with respect to fatigue strength evaluation.

As usual with other materials the constant-amplitude test (Wdhler) is widely adopted as the criterion for the fatigue sensitivity of concrete. In this test the number of load repetitions Ni the material can stand before failure occurs, is determined. In general stresses vary in a more erratical and random way. As for concrete from tests no relation is known between this kind of loading and the service life of a structure, Miner's rule is adopted for predicting this life on basis of constant-amplitude tests. According to this rule failure will occur if the following condition is satisfied: where c is the number of stress cycles during the service life. As the number of stress repetitions N. is a stochastic quantity, Miner's rule will also be stochastic. An experimental verification of the rule is therefore complicated. So a theory is giventose-parate in Miner's rule the possible uncertainty in the rule from the influence of the dispersion in N. With the aid of this theory test results of about 220 variable-amplitude tests have been verified. Miner's rule proved to be very accurate to predict the life-time of the test cylinders. At the moment the test program is continued with random loading tests. The results of that will soon be available.

Results are presented from a study carried out at the Building Research Establishment, United Kingdom, on the effect of rate of‘compressive loading on the fatigue characteristics of plain concrete. Specimens made with a typical normal weight aggregrate, gravel, and a manufactured lightweight aggregate, Lytag, were tested at one of two constant rates of stressing and unstressing, 0.5 MN/m2s and 50 MN/m2s. Using as a basis the static strength of the concrete at a standard stressing rate (0.25 MN/m2s), two distinct S-N curves were obtained for each type of.concrete. When related to the static strength at the same stressing rate as in the corresponding fatigue test, the results fell onto a single curve for each type of concrete. In studying the development of axial strains in the concrete during a fatigue test, it was found that the rate of strain increase per cycle of load was constant for most of the life of the specimen. It is shown that a strong correlation exists between this strain rate and the fatigue life of the specimen, and that this can be used to determine relationships between the strain rate and the true level of loading. Using these re1ationships'S-N curves are produced with very little scatter of results.

Fifty concrete beams reinforced with 24 mm deformed bars in the tension zone, were subjected to sinusoidal load fluctuations at 6.7 Hz in air, 3 percent sodium chloride solution and natural sea water. Total numbers of cycles at failure varied between 10' and lo7 for calculated stress ranges in the steel between 100 MPa and 280 MPa. Two types of tension reinforcement were compared; one was a hot-rolled 230 Grade deformed bar, and the second a cold.-worked 410 MPa Grade deformed similar chemical composition. bar with a As practised in some countries, cold-working by twisting was found to reduce the fatigue endurance of the deformed reinforcement in concrete beams tested both in air and sea water. The detrimental effect of sea water or sodium chloride solution gaining access, via concrete cracks, to bars subjected to fatigue loading was confirmed. In sea water the influence of cyclic loading on the hot-rolled series was different to that on the cold-worked series; for the latter series a decrease in slope of portion of the S-N curve was observed, which may represent a fatigue limit within lo7 cycles, whereas for the former no such change in slope exists. A reduction of fatigue endurance was observed for tests in a 3 percent sodium chloride solution compared with data for beams loaded in natural sea water. when A fractographic investigation was conducted on typical failure surfaces of bars subjected to tests in concrete,in air, and in sea water.

Nine fiber-reinforced-concrete spheres were subjected to external pressure loading; four spheres were tested to failure by static loading, and five by low-cycle fatigue loading. The state of stress in the wall of the spheres was multiaxial, varying from biaxial on the inside surface to triaxial elsewhere. The average triaxial state of stress was 0 1 = a2 and 0 3 = 0.3 CT . 1 The fatigue data show a substantial difference in behavior compared to that of previous work on confined concrete. Changing the stress levels, c 9 from 0.70 to 0.50 changed the cycles to failure from 10 to 346, respectively. However, a better parameter to describe fatigue behavior was the stress-to-strength ratio, (qcyc/f;' which varied from 1.55 to 1.08, respectively. Under triaxial compression, it appeared that cyclic loading in which all principal stresses cycle was a considerably more severe condition than cyclic loading in which only one principal stress cycles while the other two principal stresses remain constant.

Bond-slip failure between a smooth steel bar and con-crete was studied in micro scale. Both theoretical and experimen-tal work are presented. The theoretical analysis was perfomed as a nonlinear fi-nite element analysis, based on fracture mechanics. Roth a virgin loading stageanda subsquentloadingstagewasmdelled. In the experimental part a total of ten pull-out tests were performed, both with monotonic loading and cyclic loading. As far as the load-slip relations are concerned, the specimens were much softer at virgin loading compared to subsequent repeated loading. This is suggested to be caused by the phencmenon that the concrete, when it was cast, did not completely grow into the irregularities of the steel. The gaps, which consequently arised, were determined to be of the same magnitude as the depth of the irregularities, and were causing a soft performance for shear movements at virgin loading. This softness also caused the shear stresses in the contact zone to be quite uniformly distributed along the anchorage length. At subsequent repeated loading, as far as shear movment in the same directionas the virgin loading is concerned, all gaps were filled up, which resulted in a much stiffer performance.

The results show that (a) the flexural fatigue life of normal concrete beams is higher than the fatigue life of layered beams utilizing dense concrete or latex modified concrete at lower stress levels but is lower than that of hybrid layered beams at higher stress levels and (b) essentially the same fatigue strength was obtained for beams containing only a 2-inch layer of either dense concrete or latex modified concrete at the surface as for beams containing these concretes full depth.

This paper presents results from an experimental investigation in which thirteen rectangular reinforced concrete beams were subjected to cyclic loading within service conditions. One beam was designed to measure the variation in steel stress and bond stress along the beam length, particularly between cracks and in the anchorage zones near the beam ends. The variables in the other twelve beams included the area of concrete concentric with each tension rebar and the ratio of compression reinforcement to tension reinforcement. Resulting bond deterioration is analyzed and its effect on bond stress distribution as well as crack development is illustrated and discussed.

The objective of this investigation was to study ex-perimentally the behavior of plain concrete subjected to slow cyclical loading in compressive uniaxial and biaxial states of stress. A triaxial testing machine which had been successfully used to study the response of plain concrete cubicai specimens subjected to static multiaxial stress states was used in this study. The test specimens were subjected to prescribed stress histories which included 15 maximum stress levels. In addition to the uniaxial state of stress, biaxial stress states of two types were studied. The first was a proportional loading type in which two loading paths were used, namely 02/q = I .O and 02/01 = 0.5. The second loading type consisted of a constant stressing the direction with a cyclical loading in the 01 direction. The cyclical loading ranged from zero to various percentages of the unconfined static strength of the cube. Cycle rates used were in the range of one- cycle per minute. Strain measurements in all three principal directions were recorded for each test. This data was used to show the effects of number of cycles, load paths, and state of stress on the dilatational, as well as stress-strain response for plain concrete. The data waslso used to show the effect of state of stress and load path on the fatigue strength of concrete.

The few existing tests with stress reversals between tension and compression suggest that these may have little influence on the fatigue strength of concrete. This would instead be determined by the higher of the tensile or compressive stress maxima, expressed as percentages of the appropriate static strengths, with the minimum stress equal to zero. Two series of tests were performed to gain moreinformation. The specimens used were cubes and prisms loaded with compressive loads and transverse splitting line loads. Combination of these loads and pulsation of one of them gave the desired stress reversals. The test results indicate that stress reversals cause a slight reduction in fatigue strength. This reduction may however be due to the test equipment In the Swedish Code of Practice, a reduction of design stress due to fatigue caused by stress reversals between tension and compression is required. The results obtained are on the safe side of this requirement.